Heat oven to 325 degrees F. Line an 8-inch square pan with parchment, leaving overhang on two sides for easy lifting.

In a medium saucepan over low heat, melt butter. Remove from heat and whisk in black cocoa until completely smooth and no streaks remain. The mixture will look thick and glossy, almost like melted dark chocolate.

Whisk in both sugars until fully incorporated. The mixture will look grainy at first, then become thick and shiny. Let cool 5 minutes so you do not scramble the eggs.

Whisk in eggs one at a time, beating well after each. Stir in vanilla. The batter should now be smooth and pour like thick pancake batter.

Switch to a rubber spatula. Add flour and salt, folding until just combined and no white streaks remain. Do not overmix - a few small lumps are fine. The batter will be thick and nearly black.

Scrape into prepared pan and smooth the top. Bake 26-30 minutes until the top looks set and a toothpick inserted in the center comes out with a few moist crumbs clinging to it. The center should still feel slightly soft when gently pressed.

Cool in pan on a wire rack until completely cold, at least 2 hours. The brownies firm up as they cool. Lift out using parchment overhang and cut into 16 squares with a sharp knife, wiping clean between cuts.